Method and composition for making ceramic parts by stereolithophotography and use in dentistry

ABSTRACT

The invention concerns a method wherein a pasty composition comprising a plasticizing agent is used in a selected amount to eliminate or reduce to a minimum stresses generated in the part during polymerization. The invention is useful in particular for making parts in dentistry.

The invention concerns making ceramic piece by stereolithography (rapidprototyping).

When making a ceramic piece by stereolithography, a thin layer of acomposition containing a mixture of a ceramic powder, a photocurableresin, a photoinitiator, a dispersant and possible adjuvants isdeposited on a support, this layer is cured in one or several selectedzones by the action of suitable radiation, a new layer is deposited onthe layer thus treated, and the operations are recommenced until all ofthe cured parts constitute the desired piece in the unprocessed state,the uncured piece are eliminated, the organic constituent of theunprocessed piece is eliminated, in particular by thermal decomposition(debinding), and the piece is sintered.

The method is implemented using a liquid or pasty composition.

In the case of a liquid composition, the support is immersed in a bathof the composition in such a way that it is covered only by said thinlayer, and it is lowered gradually into the bath as the layers form.

A typical example is described in the publication U.S. Pat. No.5,496,682 which indicates that the liquid must have a viscosity of lessthan 3 Pa·s, preferably less than 1 Pa·s.

In the case of a pasty composition, a suitable quantity of compositionis deposited on the support in order to be spread across the latter byraking and form the desired layer, and the operation is repeated layerby layer.

A typical example is described in the publication FR 2 88 268.

Typically, the pasty composition has a viscosity of several hundred toseveral thousand Pa·s.

The piece obtained by stereolithography starting from a liquidcomposition are soft and have to undergo subsequent UV treatment (U.S.Pat. No. 5,496,682) in order to harden them and avoid their deformationduring firing, whereas the piece obtained by stereolithography startingfrom a pasty composition have a total cure rate which induces a veryrigid architecture since the grains cannot reorganize; the result ofthis is that there are very high stresses inside the piece duringdebinding, the polymer chains degrade, and the sudden release of thesestresses can cause cracks during sintering, especially when the piecehas a wall thickness of several millimeters.

The present invention aims to resolve this problem of cracking whicharises in the case of stereolithography using a pasty solution.

In practice, these internal stresses existing in the raw piece arerelaxed during debinding and generate cracking of the piece duringsintering, especially when the piece has a wall thickness of severalmillimeters.

Surprisingly, it has been found that this problem could be resolved byincorporating a plasticizing agent into the paste, and that it was thenpossible to obtain, without any cracking, piece having a wall thicknessof over 1 centimeter.

Incorporating a plasticizing agent into a composition for making aceramic piece by rapid prototyping has already been proposed(publication U.S. Pat. No. 5,496,682), but this involved a method ofstereolithography applied to a liquid composition, and the plasticizingagent had no function other than to reduce the viscosity of thecomposition so that the composition remains liquid.

This use could not therefore suggest using a plasticizing agent in acomposition which is to remain pasty, and what is more to resolve aproblem which only arises in the case of a pasty composition.

According to the present invention, the addition of plasticizing agentto the pasty composition is advantageously combined with the use of aquantity of ceramic powder sufficient to maintain the viscosity of thecomposition at a value of at least 10 000 Pa·s at a flow velocitygradient of 0.1 sec⁻¹ such that the paste is “self-holding”, that is tosay does not flow by itself, and that the uncured parts of one layer cansupport the following layer.

According to the invention, the plasticizing agent eliminates or reducesto a minimum the internal stresses during photocuring and permits rapidrelaxation of the possible residual stresses which may appear afterexposure to radiation and are due to the curing kinetics, this by virtueof a reduction in the interactions between the chains, which favorstheir mobility.

The term “ceramic powder” designates one or more ceramic powders.

The word “resin” designates one or more resins.

In preferred embodiments, one or more of the following characteristicsare implemented:

-   -   a pasty composition is used which comprises an alumina charge        with a rate of at least 58% by volume of the volume of the        composition;    -   a pasty composition is used which comprises an alumina charge        with a rate of between 60 and 70% by volume of the volume of the        composition;    -   a pasty composition is used which comprises an alumina charge        with a rate of about 62-63% by volume of the volume of the        composition;    -   a composition is used having an elastic modulus greater than the        viscosity modulus;    -   a pasty composition is used which comprises a zirconia charge        with a rate of 49 to 55% by volume of the volume of the        composition;    -   a trifunctional photocurable resin is used;    -   an acrylate resin is used as photocurable resin;    -   an acrylate resin is used from the group formed by:        -   di-ethoxylated bisphenol A dimethacrylate (for example            Diacryl 101 from AKZO),        -   1,6-hexanediol diacrylate (for example HDDA from UCB);    -   a pasty composition is used which comprises 20 to 50% by volume        of plasticizing agent relative to the volume of the resin;    -   a pasty composition is used in which the plasticizing agent is        one or more agents from the group formed by the family of        glycols (e.g. polyethylene glycol), the family of phthalates        (e.g. dibutylphthalate), glycerol.

Curing of acrylates is initiated through absorption of ultraviolet lightby substances generating free radicals. The initiators of the acrylatesare of the cationic type and their choice is guided principally by thewavelength of the light source they have to absorb, i.e. 350-360 mm inthe case of ultraviolet.

Two particularly effective photoinitiators are the following:

-   -   2,2′-dimethoxy-2-phenylacetophenone (for example Irgacure 651        from CIBA)    -   2-hydroxy-2-methyl-1-phenyl-propan-1-one (for example Darocure        1173 from CIBA).

The dispersant used must be compatible with the photocurable resin inwhich it is dissolved and must be effective with the ceramic powder tobe dispersed. The polyelectrolytes used in other ceramic processes areunsuitable because they do not easily dissociate in this type of medium.Dispersants with steric or electrosteric stabilization mechanisms arepreferred.

Phosphoric esters have proven to be good dispersants.

Any plasticizing agent compatible with resins may be envisaged, inparticular polyethylene glycol and glycerol in the case of acrylateresins. Dibutyl phthalate proves less effective.

EXAMPLES

Pastes are prepared comprising (% by volume of the total volume): PasteA Paste B Ceramic (1) 62 59 Resin (2) 29.6 28.4 Photoinitiator (3) 0.1 1Dispersant (4) 4.7 4.5 Plasticizer (5) 6.3 7.1

The viscosity of the paste at 0.1 sec⁻¹ is 14 200 Pa·s (paste A) and 13200 Pa·s (paste B).

-   (1) alumina in paste A and hydroxyapatite oxyhapatite in paste B,-   (2) Diacryl 101 from AKZO (paste A) and CN 503 from CRAY VALLEY    (paste B),-   (3) Irgacure 651 from CIBA,-   (4) Beycostat A 259 from CECA (paste A) and C213 (paste B),-   (5) PEG 300 from Merck.

Using paste A, rapid prototyping is performed with 188 layers of 100microns to make a grille-shaped piece with overall dimensions of230×230×13.8 mm, this piece being subjected to thermal treatment(debinding) up to 600° C., with a holding time of 2 hours at 600° C.,then to sintering up to 1700° C., with a holding time of 1 hour 30minutes at 1700° C.

The piece obtained has a flexural strength of 396 MPa.

Using paste B, rapid prototyping is performed with 230 layers of 100microns to make a piece with overall dimensions of 72×37×23 mm, thispart being subjected to thermal treatment (debinding) up to 600° C.,with a holding time of 2 hours at 600° C., then to sintering up to 1400°C., with a holding time of 1 hour 30 minutes at 1400° C.

The piece obtained has a flexural strength of 102 MPa.

By way of comparison, a ceramic piece was produced by the bath techniqueusing a liquid composition and where the ceramic charge rate is only46.4% by volume: ceramic: 151 cm³  resin: 91 cm³ photoinitiator:  5 cm³dispersant: 39 cm³ wetting agent: 7.2 cm³  plasticizer: 32 cm³

Despite the low ceramic charge rate, preparation is difficult andnecessitates the use of a solvent. The curing rate is very low.Production of a simple piece using this composition is possible only ifit is of small size and without geometric detail (small rod, cube,cylinder). The unprocessed piece is very soft and deforms easily.Appearance of some delamination. After debinding and sintering, a smallrod measuring 0.5×0.5×2 cm is completely fissured.

Among the possible applications of the invention, particular mentionought to be made of the application in dentistry.

Most dental bridges presently consist of a metal cap, which may or maynot be covered with a porcelain. The latter, consisting of differentceramic layers which have been successively fired at high temperature,makes it possible to give the desired shade to the tooth or to all theteeth (bridge) to be implanted, so as to permit perfect integrationthereof with the patient's dentition.

The dental structures are subjected to high mechanical stresses duringtheir use, and the metal part is able to satisfy these demands. The maindisadvantage, however, lies in the fact that several ceramic layers arenecessary for:

-   -   masking the metal of the cap and giving good translucence and        coloring;    -   ensuring the coefficients of dilation with the aim of obtaining        a stable system free from microfissures.

The longevity of such systems depends on the quality of the bond betweenthe metal and the first ceramic layer. The interface between these twomaterials is the source of defects, for example fissures.

With the aim of increasing the longevity of dental structures, of makingcoloration of the teeth easier, while at the same time limiting thenumber of layers generally of porcelain, the metal ring can be replacedby a ceramic ring.

The method and the composition of the present invention make it possibleto obtain ceramic dental piece of small dimension, but with very precisedimensions adapted to each patient.

In this application, use will preferably be made of a stabilizedzirconia powder, for example a zirconia stabilized with 3 mol % yttria(Y₃O₂). This stabilization makes it possible to retain a tetragonalmicrostructure and avoid any phase change causing fissuring of denseparts.

The stabilized zirconia has good mechanical properties (1200 MPa in 3point flexion (supplier's data)), especially when the grainsconstituting the powder are very fine (<0.5 μm).

Production of the ceramic paste must be mastered in order to control thephenomena of rheology and reactivity. The use of fine zirconia powderallows the suspension to be charged with levels of between 49 and 55%.The viscosity at a flow velocity gradient of 0.1 is of the order of 10000 to 15 000 Pa·s.

The production of dental structures requires the formation of ceramicpiece having good tolerance properties. To do this, fine layers areformed during production. Their thickness is 25 um, making it posible toobtain a good surface state and greater precision. These low thicknessesare necessary for retaining reasonable curing speeds. This is becausethe low reactivity of zirconia-based pastes is a limiting factor withregard to the quantity of the pieces to be produced. The decrease inthickness of the layers makes it possible to reduce the curing time.

Working with such small thicknesses imposes more constraints duringlayer formation, and defects may be generated, such as local tears, lackof ceramic paste in places, etc. Organic products such as rheologyagents make it possible to spread the paste in small thicknesses andobtain a correct layering. It is also possible to improve the surfacestate of each layer by addition of wetting agents or antifoaming agents,which products have the particularity of degassing the paste and ofenhancing its spread during layering. These compounds make it possibleto eliminate the defects at each layer.

The reactivity of the ceramic paste is an important parameter. Inaddition to the resin, it is possible to influence this characteristicby using a suitable photoinitiator or a mixture of photoinitiators. Theaim of this is to be able to cure sufficient thicknesses to produce thedesired part.

The composition advantageously comprises rheology agents so that thelayers have surface states without any defects susceptible of creatingmicroporosities.

An example of a ceramic paste is given below (% by volume of the totalvolume): ceramic powder: ZrO₂ (49%) resin: CN5O3 from CRAY VALLEY(30.6%) photoinitiators: Irgacure 369 from CIBA Irgacure 819 from CIBAIrgacure 907 from CIBA dispersant: Beycostat C213 from CECA (5.5%)plasticizer: Dibutylphthalate from Acros Organics (4.6%) rheologyagents: Rad 2100 from Tego (2.8%) Rad 2500 from Tego (2.9%) Glide 450from Tego (2.9%) antifoaming agent: Foamex N from Tego (2.9%)

Rapid prototyping technology adapted to dental structures permitstailor-made production of a cap or a bridge needed for a given patient.These structures can be simple individual caps or an assembly of caps(bridges) whose shape can be straight or curved.

A scanned image of the part to be repaired is processed using suitablesoftware (CAO) making it possible to redimension the ceramic piece to beproduced and cut it into successive sections of 25 μm.

The piece is then constructed physically by rapid prototyping with thepaste described above.

Using this paste, rapid prototyping is performed to produce bridgesmeasuring 40×6×12 mm³ and, after the uncured paste has been cleaned,these are subjected to thermal treatment (debinding) up to 550° C., witha holding time of 2 hours, then to sintering up to 1400° C., or even1550° C., with a holding time of 2 hours, depending on the zirconiaused.

The piece obtained has a flexural strength of the order of 1000 MPa.

In other embodiments, the zirconia is doped with alumina to increase themechanical properties.

The invention is not limited to these examples.

1. A method for making a ceramic piece by a rapid prototyping procedure,which comprises operations in which a thin layer of a pasty compositioncontaining a mixture of a ceramic powder, a photocurable resin, aphotoinitiator, a dispersant and possible adjuvants is deposited on asupport, this layer is cured locally in one or several selected zones bythe action of suitable radiation, a new layer is deposited on the layerthus treated, and the operations are recommenced until all of the curedparts constitute the desired piece in the unprocessed state, the uncuredparts are eliminated, the organic constituent of the unprocessed pieceis eliminated by thermal decomposition (debinding), and this piece issintered, characterized in that a plasticizing agent is incorporatedinto the composition in a selected quantity relative to the quantity ofcurable resin in order to eliminate or reduce to a minimum the stressesgenerated in the piece during curing so as to avoid cracking of thepiece during sintering, and in that a composition is used containing aceramic powder in a sufficient quantity to ensure that the compositionhas a viscosity of at least 10 000 Pa·s at a flow velocity gradient of0.1 sec⁻¹, such that the paste is “self-holding”, that is to say doesnot flow by itself, and that the uncured parts of one layer can supportthe following layer.
 2. A pasty composition for making a ceramic pieceusing the method as claimed in claim 1, which composition comprises aceramic powder, a photocurable resin, a photoinitiator, a dispersant andpossible adjuvants, characterized in that the composition comprises aplasticizing agent in a selected quantity to eliminate or reduce to aminimum the stresses generated in the piece during curing, so as toavoid cracking of the piece during sintering, and in that the proportionof ceramic powder is sufficient to ensure that the composition has aviscosity of at least 10 000 Pa·s at a flow velocity gradient of 0.1sec⁻¹, such that the paste is “self-holding”, that is to say does notflow by itself, and that the uncured parts of one layer can support thefollowing layer.
 3. The composition as claimed in claim 2, which has anelastic modulus greater than the viscosity modulus.
 4. The compositionas claimed in one of claims 2 and 3, which comprises from 20 to 50% byvolume of plasticizing agent relative to the volume of the resin.
 5. Thecomposition as claimed in one of claims 2 through 4, in which theplasticizing agent is one or more agents from the group formed by thefamily of glycols, the family of phthalates, and glycerol.
 6. Thecomposition as claimed in one of claims 2 through 5, which comprises, asphotocurable resin, a trifunctional resin.
 7. The composition as claimedin one of claims 1 through 6, and which comprises, as photocurableresin, an acrylate resin.
 8. The composition as claimed in claim 7, inwhich the photocurable resin is a resin from the group formed by thefollowing resins: di-ethoxylated bisphenol A dimethacrylate1,6-hexanediol diacrylate.
 9. The composition as claimed in one ofclaims 2 through 8, which comprises at least 58% (by volume) of ceramicpowder relative to the total volume of the composition.
 10. Thecomposition as claimed in claim 9, which comprises from 60 to 70% (byvolume) of ceramic powder.
 11. The composition as claimed in claim 10,which comprises about 62-63% (by volume) of ceramic powder.
 12. Thecomposition as claimed in one of claims 9 through 11, in which theceramic powder is an alumina powder.
 13. The composition as claimed inone of claims 2 through 8, in which the ceramic powder is a stabilizedzirconia powder.
 14. The composition as claimed in claim 13, and whichcontains 49 to 55% by volume of zirconia powder relative to the totalvolume of the composition.
 15. The composition as claimed in claim 14,in which the zirconia is doped with alumina.
 16. Use of the method asclaimed in claim 1 and of the composition as claimed in one of claims 2through 15 for producing caps, bridges and other dental structures. 17.Use as claimed in claim 16, in which the method is implemented usinglayers with a thickness of not more than 25 micrometers.